Traditional telephone networks comprise switched networks, e.g., the public switching telephone network (PSTN) and ISDN lines, in which a physical link path is established between the end users of a call. The voice signals of the call are passed, generally in a time domain multiplexed (TDM) manner, on the physical link path. On the other hand, data networks, such as the Internet, are generally packet based networks.
Due to the wide availability of switched network lines, these lines are commonly used to access packet based networks.
FIG. 1 is a schematic illustration of use of a switched network 10 in carrying voice band modem (VBM) connections, as is known in the art. A pair of modems 12 (marked 12A, 12B, 12C and 12D) form a VBM connection with each other through the PSTN 14. In establishing the connection, the pair of modems 12 perform a negotiation stage in which the parameters test the line and determine operation parameters, e.g., a data rate and/or protocol, according to which data will be transferred over the connection. Thereafter, a data transfer stage is performed at which the modems modulate and demodulate the data packets they are provided and pass the modulated signals over the connection. As is known in the art, VBM connections may be established between client modems 12A and 12D, which are connected to PSTN 14 through twisted pairs 16, between server modems 12B and 12C which are connected directly to the infrastructure of the PSTN, or between a server modem and a client modem. The various connections may be established in accordance with various ITU recommendations, such as V.34, V.90, V.91 and V.92.
Existing PSTN hardware cannot support the increasing demand for all types of communication services, and therefore additional hardware is added to the PSTN instead of, or in addition to, the current hardware of the PSTN. In many cases, packet based networks are cheaper to install and maintain than switched networks. Therefore, telephony providers are adding many packet based lines and/or networks to their infrastructure, especially to their long distance calls infrastructure.
FIG. 2 is a schematic diagram of a telephone connection 20 known in the art, which includes a segment on a packet based network 26 as well as segments on switched networks 22. Voice signals transferred from an end unit 24A (e.g., a telephone set) are passed along switched network 22A, using standard telephony methods, to a gateway 28A. Gateway 28A packs the stream of signals from end unit 24A into packets and passes the packets over packet based network 26 to a second gateway 28B. Gateway 28B unpacks the received packets and passes them in a stream on switched network 22B to an end unit 24B. A similar process is performed on signals transferred in the opposite direction.
The travel time of packets through packet based network 26 may vary for different packets of a single connection. Therefore, gateway 28B maintains a buffer which delays the packets it receives for a short period allowing the gateway to organize the packets in their original order. When packets are not received within the short delay, gateway 28B generates filler signals to replace the delayed packet. If only a small percentage of packets are delayed beyond the allowed period (or are otherwise lost), telephone conversations are passed with sufficient quality such that the loss of packets is substantially unnoticed.
The telephone network is used, in addition to voice exchange, for other services, such as fax and data exchange using voice band modems (VBM). In fax connections, image data is modulated onto voice signals, which are transmitted over switched networks 22 together with control information. The loss of a relatively small percent of the modulated signals is sufficient to prevent demodulation of the signals. Therefore, when gateway 28A receives fax signals, the gateway demodulates the signals and passes them to gateway 28B in a predetermined packet format, for example as described in the T.38 ITU-T recommendation, “Procedures for real-time Group 3 facsimile communication over IP networks”, the disclosure of which is incorporated herein by reference. Gateway 28A identifies the fax signals using methods such as described, for example, in U.S. Pat. No. RE35,740, the disclosure of which is incorporated herein by reference. Gateway 28B extracts the fax data from the packets and re-modulates the fax data for transmission on links of switched network 22B.
It is noted that most VBM connections are local connections to Internet service providers (ISP), which provide gateway services to packet-based networks. Nonetheless, there are cases when it is desired to create long distance modem connections, for example, for remote access (RAS) applications.
VBM sessions are formed of a negotiation stage in which the end-modems of the session choose parameters of the session, such as the protocol and data rate, and a data transmission stage in which data is exchanged between the modems.